The present invention is directed to the field of ignition devices for combustible fluids and is more particularly directed to hot surface ignitors.
Ignition systems for combustible fluids are used in many different applications. One well-known application is for ignition of gas in a furnace.
Typically, a temperature sensor measures the temperature of a known space. As shown in FIG. 1, a thermostat 105 may receive a temperature signal from the temperature sensor and compare the temperature value represented by the signal to a stored desired temperature. If the temperature signal is below the desired temperature, the thermostat may cause a furnace 110 to start.
Referring now to FIGS. 1 and 2, the furnace uses an ignition system, such as a hot surface ignitor 112A, to ignite gas in the furnace at start-up. In the past, a relay 112B has closed at start-up of the furnace, the closure causing gas to be released in the furnace and heating of hot surface ignitor. The hot surface ignitor is powered by a power supply 115.
U.S. Pat. No. 4,978,292 issued on Dec. 18, 1990 to Donnelly et al. (the '292 patent) and 4,925,386 issued on May 15, 1990 to Donnelly et al. (the '386 patent) teach modified ignition circuits and methods. In particular, a triac was used in place of the relay in the ignition circuit and a flame detector was used to provide feedback. The triac was switched on and off by a microprocessor. The microprocessor operated to tightly control the operating voltage of the ignitor to the minimum required level which achieved ignition of the gas. The microprocessor would control the power reaching the ignitor by limiting the on-time of the triac. On start-up, an on-time was picked which was more than sufficient to ignite the chosen fuel. At successive start-ups, the on-time was shortened until the flame detector did not detect flame on a particular start-up. The on-time was then lengthened back to a point which was known to cause a flame.
Still, a problem existed with using even a triac in the switching of the ignitors in which their impedance increases as their temperature increases. This is true in silicon nitride ignitors in particular. When the ignitor is cold, its resistance is very low. This resistance increases as the ignitor warms. When the relay closed and energized the cold ignitor, a large load appeared on the system transformer. The shaded area of FIG. 2A represents the on time of the hot surface ignitor compared to the supply voltage (which is 100% here). This temporarily pulled down the system transformer output voltage which occasionally caused low voltage-related problems for other components connected to the system transformer.